This invention relates to the supervision of communication lines and, more particularly, to the use of randomly generated signals for detecting breaches in the security of a communication system.
Although alarm systems for the detection of fire and intrusions are well known in the art and although it is well known to monitor the integrity of the line running from the alarm indicator to the end of line element in order to assure that the line is not grounded or opened thus breaching security, the increased sophistication of the criminal makes it necessary to achieve a higher level of integrity supervision. Without supervision, it is a relatively easy matter for anyone who wishes to breach the security of the monitoring system to simply substitute an end of line device. However, wiring around the end of line device either adds or subtracts resistance from the alarm loops such that early integrity supervising systems merely monitored the current level of the alarm loop for detecting any changes in resistance of the line. However, if the limits of current to which the central monitor responds are known, it is a simple matter to insure that the current on the line remains the same when the security system is breached. Thus, there is a need for developing even more sophisticated integrity supervision systems.
The modern fire and security system typically comprises a computerized central processing and monitoring unit which digitally communicates with a plurality of remote stations each of which may have several alarm loops. When one of the loops in a remote station senses a fire or security condition, it raises an alarm flag, which may be a change of status signal, such that when the remote station is next polled by the central processing unit, the remote station will transmit its alarm information. During the polling operation, the central processing unit typically transmits a corresponding address to each of the remote stations requesting them to supply any information if any of their alarm loops have undergone a change in status since the last poll. The central processing unit will then display this alarm information on a printer or other form of visual display and may also provide an audible warning of the alarm condition. The remote station detects alarm conditions by monitoring the line current on each of its loops. Thus, the remote station is able to detect alarm conditions as well as open and short conditions on the line.